1. Field of the Invention
The present invention relates to a ball bearing and, in particular, to a double row ball bearing suitable for use in an engine cooling water pump in which raceways capable of holding balls are of a double row type and a counter bore is formed in one of two raceways of an outer race. Also, the present invention relates to an improvement in the endurance of the double row ball bearing.
2. Description of the Related Prior Art
FIG. 5 is a partially longitudinal section view of a conventional-type water pump double row ball bearing 30 suitable for use in a water pump which is used to circulate a coolant for an engine of a car. This type of double row ball bearing is disclosed in, for example, Japanese Patent Unexamined Publication No. Hei. 9-22212.
In FIG. 5, in a rotary shaft 31 which corresponds to an inner race, there are formed a first inner race raceway 32 consisting of a groove and a second inner race raceway 33 also consisting of a groove in such a manner that they respectively extend over the whole circumferential direction of the outer peripheral portion of the rotary shaft 31.
On the other hand, in the inner peripheral portion of a substantially cylindrical-shaped outer race 34, there are formed a first outer race raceway 37, which is composed of a groove 35 and a counterbore portion 36 having a linear-shaped cross section, and a second outer race 38 composed of a groove 38 in such a manner that they respectively extend over the whole circumferential direction of the inner peripheral portion of the outer race 34.
The outer race 34 is disposed in such a manner that the center line C thereof is identical with the center line C' of the rotary shaft 31, the first inner race raceway 32 is opposed to the first outer race raceway 37, and the second inner raceway 33 is opposed to the second outer race raceway 38. Also, the shapes and clearances of the first and second raceways are selected in such a manner that they substantially correspond to the shapes of balls 39 to be incorporated between the corresponding raceways and also the ball 39 incorporated between the corresponding raceways are allowed to roll freely there.
The balls 39 are incorporated between the pair of first inner and outer race raceways in the following manner. That is, a plurality of balls 39 are incorporated around the circumferential direction of the first inner and outer race raceways, in more detail, in a so called maximum manner in which as many balls 39 as possible can be incorporated. By the way, the balls 39 are uniformly distributed at a predetermined interval in the circumferential direction thereof and can be held by a tilt-type retainer 40.
On the other hand, between the second inner and outer raceways, there are incorporated balls 41 in the following manner. That is, a plurality of balls 41 are incorporated around the circumferential direction of the second inner and outer raceways, in more detail, the balls 41 are incorporated in a proper number which is smaller than the number of balls 39 to be incorporated into and between the pair of first inner and outer raceways. By the way, the balls 41 can be uniformly distributed at a predetermined interval in the circumferential direction thereof and can be held by a retainer 42.
On the two end portions of the outer race 34 in the center line direction thereof, there are disposed sealing devices 43 and 44 in order not only to prevent leakage of bearing lubricant but also to prevent invasion of liquid into the bearing from the outside. Thus, the portions of the ball bearing 30 where the balls 39 and 41 are incorporated are sealed against the outside by the sealing devices 43 and 44.
The thus structured ball bearing has an advantage that, since the formation of the counterbore portion can increase the number of balls to be incorporated into one row of raceways thereof, the resistance of the present raceways to a radial load can be increased.
By the way, as known well, generally, a rolling bearing is roughly classified into a ball bearing and a roller bearing according to the kinds of rolling elements used. And, normally, there is a tendency that the roller bearing has larger resistance to a rated radial load (which is hereinafter referred to as a rated load) in the axial direction than the ball bearing. Therefore, in the above-mentioned double row ball bearing for a water pump, use of rollers as the rolling elements thereof is preferable from the viewpoint of enhancement in the rated load.
However, when the roller is used as the rolling element, if the rotary shaft is inclined relatively to the outer race, then there is applied an excessive load (which is referred to an edge load) onto and between the end edges of the rolling surfaces of the respective rollers and the outer and inner races, which lowers the fatigue life of the inner and outer races. And, the narrower the width of the outer race is, the easier the edge load is to occur. This deteriorates the ratio of the actual life of the outer race to the calculated life thereof. Therefore, it can be concluded that, especially when the outer race has a short pitch, it is not preferable to use the roller as the rolling element of the rolling bearing.
Thus, it is necessary to employ a double row bearing which uses balls as rolling elements thereof. In this case, as in the water pump double row ball bearing shown in FIG. 5, there may be employed a structure in which a counterbore portion is formed in one of the two outer race raceways to thereby increase the number of balls to be incorporated into the present outer race raceway and thus increase the rated load of that portion.
However, there is a limit to the size of the ball used as the rolling element. For example, even when the number of balls can be increased, if the diameter of each ball is not large to a certain degree, it is impossible to achieve a satisfactory rated load. On the other hand, when the ball diameter is excessively large, the number of balls that can be incorporated decreases, which also deteriorates the rated load of the bearing. Therefore, with employment of this structure, a condition for the replacement of the roller by the ball cannot be satisfied.